It is well known to include in various kinds of imaging elements, a transparent layer containing magnetic particles dispersed in a polymeric binder. The inclusion and use of such transparent magnetic recording layers in light-sensitive silver halide photographic elements has been described in U.S. Pat. Nos. 3,782,947; 4,279,945; 4,302,523; 5,217,804; 5,229,259; 5,395,743; 5,413,900; 5,427,900; 5,498,512; and others. Such elements are advantageous because images can be recorded by customary photographic processes while information can be recorded simultaneously into or read from the magnetic recording layer by techniques similar to those employed for traditional magnetic recording art.
A difficulty, however, arises in that magnetic recording layers generally employed by the magnetic recording industry are opaque, not only because of the nature of the magnetic particles, but also because of the requirements that these layers contain other addenda which further influence the optical properties of the layer. Also, the requirements for recording and reading of the magnetic signal from a transparent magnetic layer are more stringent than for conventional magnetic recording media because of the extremely low coverage of magnetic particles required to ensure transparency of the transparent magnetic layer as well as the fundamental nature of the photographic element itself. Further, the presence of the magnetic recording layer cannot interfere with the function of the photographic imaging element.
The transparent magnetic recording layer must be capable of accurate recording and playback of digitally encoded information repeatedly on demand by various devices such as a camera or a photofinishing or printing apparatus. The layer also must exhibit excellent runnability, durability (i.e., abrasion and scratch resistance), and magnetic head-cleaning properties without adversely affecting the image quality of the photographic elements. However, this goal is extremely difficult to achieve because of the nature and concentration of the magnetic particles required to provide sufficient signal to write and read magnetically stored data and the effect of any noticeable color, haze or grain associated with the magnetic layer on the optical density and granularity of the photographic layers. These goals are particularly difficult to achieve when magnetically recorded information is stored and read from the photographic image area. Further, because of the curl of the photographic element, primarily due to the photographic layers and the core set of the support, the magnetic layer must be held more tightly against the magnetic heads than in conventional magnetic recording in order to maintain planarity at the head-media interface during recording and playback operations. Thus, all of these various characteristics must be considered both independently and cumulatively in order to arrive at a commercially viable photographic element containing a transparent magnetic recording layer that will not have a detrimental effect on the photographic imaging performance and still withstand repeated and numerous read-write operations by a magnetic head.
Problems associated with the generation and discharge of electrostatic charge during the manufacture and use of photographic film and paper have been recognized for many years by the photographic industry. The accumulation of charge on film surfaces leads to the attraction of dust, which can produce physical defects. The discharge of accumulated charge during or after the application of the sensitized emulsion layers can produce irregular fog patterns or static marks in the emulsion. The severity of the static problems has been exacerbated greatly by the increases in sensitivity of new emulsions, increases in coating machine speeds, and increases in post-coating drying efficiency. The charge generated during the coating process results primarily from the tendency of webs of high dielectric constant polymeric film base to undergo triboelectric charging during winding and unwinding operations, during transport through the coating machines, and during post-coating operations such as slitting, perforating, and spooling. Static charge can also be generated during the use of the finished photographic product. In an automatic camera, because of the repeated motion of the photographic film in and out of the film cassette, there is the added problem of the generation of electrostatic charge by the movement of the film across the magnetic heads and by the repeated winding and unwinding operations, especially in a low relative humidity environment. The accumulation of charge on the film surface results in the attraction and adhesion of dust to the film. The presence of dust not only can result in the introduction of physical defects and the degradation of the image quality of the photographic element but also can result in the introduction of noise and the degradation of magnetic recording performance (e.g., S/N ratio, "drop-outs", etc.). This degradation of magnetic recording performance can arise from various sources including signal loss resulting from increased head-media spacing, electrical noise caused by discharge of the static charge by the magnetic head during playback, uneven film transport across the magnetic heads, clogging of the magnetic head gap, and excessive wear of the magnetic heads. In order to prevent these problems arising from electrostatic charging, there are various well known methods by which an electrically-conductive layer can be introduced into the photographic element to dissipate any accumulated electrostatic charge.
Antistatic layers containing electrically-conductive agents can be applied to one or both sides of the film base as subbing layers either beneath or on the side opposite to the silver halide emulsion layers. An antistatic layer also can be applied as an outermost layer overlying the emulsion layers or on the side opposite to the emulsion layers or on both sides of the film base. For some applications, it may be advantageous to incorporate the antistatic agent directly into the film base or to introduce it into a silver halide emulsion layer. Typically, in photographic elements of prior art having a transparent magnetic recording layer, the antistatic layer was preferably present as a backing layer underlying the magnetic recording layer.
The use of such electrically-conductive layers containing suitable semi-conductive metal oxide particles dispersed in a film-forming binder in combination with a transparent magnetic recording layer in silver halide imaging elements has been described in the following examples of the prior art. Photographic elements including a transparent magnetic recording layer and a transparent electrically-conductive layer both located on the backside of the film base have been described in U.S. Pat. Nos. 5,147,768; 5,229,259; 5,294,525; 5,336,589; 5,382,494; 5,413,900; 5,457,013; 5,459,021; 5,707,791 and others. The conductive layers described in these patents contain fine granular particles of a semi-conductive crystalline metal oxide such as zinc oxide, titania, tin oxide, alumina, indium oxide, silica, complex or compound oxides thereof, and zinc antimonate and indium antimonate dispersed in a polymeric film-forming binder. Of these conductive metal oxides, antimony-doped tin oxide and zinc antimonate are preferred. A granular, antimony-doped tin oxide particle commercially available from Ishihara Sangyo Kaisha under the tradename "SN-100P" was disclosed as particularly preferred in Japanese Kokai Nos. 04-062543, 06-161033, and 07-168293.
Preferred average diameters for granular conductive metal oxide particles in such conductive layers was disclosed to be less than 0.5 .mu.m in U.S. Pat. No. 5,294,525; 0.02 to 0.5 .mu.m in U.S. Pat. No. 5,382,494; 0.01 to 0.1 .mu.m in U.S. Pat. Nos. 5,459,021 and 5,457,013; and 0.01 to 0.05 .mu.m in U.S. Pat. No. 5,457,013. Suitable conductive metal oxide particles exhibit specific volume resistivities of 1.times.10.sup.10 ohm.multidot.cm or less, preferably 1.times.10.sup.7 ohm.multidot.cm or less, and more preferably 1.times.10.sup.5 ohm.multidot.cm or less as taught in U.S. Pat. No. 5,459,021. Another physical property used to characterize crystalline metal oxide particles is the average x-ray crystallite size. The concept of crystallite size is described in detail in U.S. Pat. No. 5,484,694 and references cited therein. Transparent conductive layers containing semiconductive antimony-doped tin oxide granular particles exhibiting a preferred crystallite size of less than 10 nm are taught in U.S. Pat. No. 5,484,694 to be particularly useful in imaging elements. Similarly, photographic elements having transparent magnetic layers in combination with conductive layers containing granular conductive metal oxide particle with average crystallite sizes ranging from 1 to 20 nm, preferably 1 to 5 nm, and more preferably from 1 to 3.5 nm are claimed in U.S. Pat. No. 5,459,021. Advantages to using metal oxide particles with small crystallite sizes are disclosed in U.S. Pat. Nos. 5,484,694 and 5,459,021 including the ability to be milled to a very small size without significant degradation of electrical performance, ability to produce a specified level of conductivity at lower weight loadings and/or dry coverages, as well as decreased optical density, decreased brittleness, and decreased cracking of conductive layers containing such particles. Conductive layers containing such metal oxide particles have been applied at the following preferred ranges of dry weight coverages of metal oxide: 3.5 to 10 g/m.sup.2 ; 0.1 to 10 g/m.sup.2 ; 0.002 to 1 g/m.sup.2 ; 0.05 to 0.4 g/m.sup.2 as disclosed in U.S. Pat. Nos. 5,382,494; 5,457,013; 5,459,021; and 5,294,525, respectively. Preferred ranges for the metal oxide content in the conductive layers include: 17 to 67% by weight, 43 to 87.5% by weight, and 30 to 40% by volume as disclosed in U.S. Pat. Nos. 5,294,525; 5,382,494; and 5,459,021, respectively.
Conductive subbing and backing layers for graphic arts films containing "short fibre", "needle-like" or "fibrous" conductive materials have been described in: U.S. Pat. Nos. 5,122,445 and 4,999,276; European Application No. 404,091; Japanese Kokai No. 04-97339. Suitable fibrous conductive materials include non-conductive fibrous TiO.sub.2 particles overcoated with a thin layer of conductive metal oxide fine particles as described in Japanese Kokai No. 59-006235. Preferred conductive fibrous particles were disclosed to exhibit average lengths of 25 .mu.m and diameters of 0.5 .mu.m, with a length:diameter ratio of about 3 or greater. Conductive backings for silver halide photographic films containing fibrous conductive metal oxides of Zn, Ti, Sn, Al, In, Si, Mg, Ba, Mo, W or V or multi-component oxides thereof coated at a dry coverage of about 0.3 g/m.sup.2 with an optional fluorosurfactant are described in Japanese Kokai Nos. 04-27937 and 04-29134. Other photographic films in which conductive K.sub.2 Ti.sub.6 O.sub.13 whiskers available from Otsuka Chemical under the tradename "Dentall WK-100S" are incorporated in subbing, backing or surface protective layers at dry coverages of 0.1-10 g/m.sup.2 are described in Japanese Kokai No. 63-98656. Silver halide laser scanner films containing conductive fibers 10 .mu.m or less in length, 0.3 .mu.m or less in diameter, and having a ratio of length to diameter of 3 or more are disclosed in U.S. Pat. No. 5,582,959. The use of conductive K.sub.2 Ti.sub.6 O.sub.13 whiskers 0.05-1 .mu.m in diameter and 1-25 .mu.m in length dispersed in the emulsion layer of such films is disclosed in Japanese Kokai No. 63-287849. Conductive coatings for photographic papers containing fibrous TiO.sub.2 particles or K.sub.2 Ti.sub.6 O.sub.13 whiskers coated with conductive antimony-doped SnO.sub.2 particles have been described in European Application No. 616,252 and Japanese Kokai No. 01-262537.
Thermal media with conductive layers containing fibrous conductive metal oxide particles 0.3 .mu.m in diameter and 10 .mu.m in length are described in Japanese Kokai No. 07-295146. Thermographic media coated with ZnO, Si.sub.3 N.sub.4 or K.sub.2 Ti.sub.6 O.sub.13 conductive whiskers are described in World Patent Application No. 91-05668.
Conductive layers for electrostatic recording films containing fibrous conductive particles are described in U.S. Pat. No. 5,116,666 and Japanese Kokai No. 63-60452. The preferred fibrous conductive particles were disclosed to be obtained commercially from Otsuka Chemical under the tradename "Dentall WK200B". Such particles consist of a thin layer of conductive antimony-doped tin oxide particles deposited on the surface of a nonconductive K.sub.2 Ti.sub.6 O.sub.13 core particle. An electrostatic recording paper having a conductive layer containing such conductive K.sub.2 Ti.sub.6 O.sub.13 whiskers also has been described in Japanese Kokai, No. 02-307551. An electrophotographic support containing rod-shaped conductive ZnO particles is described in World Patent Application No. 94-25966.
A silver halide photographic film including a transparent magnetic recording layer and a conductive backing or subbing layer containing fibrous TiO.sub.2 particles surface-coated with a thin layer of conductive antimony-doped SnO.sub.2 particles has been taught in a Comparative Example in U.S. Pat. No. 5,459,021. The average size of the fibrous composite conductive particles was about 0.2 .mu.m in diameter and 2.9 .mu.m in length. Further, the fibrous composite particles were reported to exhibit a crystallite size of 22.3 nm. Such fibrous composite conductive particles are commercially available from Ishihara Sangyo Kaisha under the tradename "FT-200". However, conductive layers containing these fibrous composite particles also were disclosed to exhibit fine cracks which resulted in decreased conductivity, increased haze, and decreased adhesion compared to similar layers containing granular conductive tin oxide particles.
The use of crystalline, single-phase, acicular, conductive metal-containing particles in transparent conductive layers for various types of imaging elements also containing a transparent magnetic recording layer has been disclosed in U.S. Pat. No. 5,731,119 incorporated herein by reference. Suitable acicular, conductive metal-containing particles were disclosed therein to have a cross-sectional diameter of 0.02 .mu.m or less and an aspect ratio (length to cross-sectional diameter) greater than or equal to 5:1. An aspect ratio greater than or equal to 10:1 was disclosed to be preferred.
However, there is a common deficiency in the electrical performance of conductive layers containing either granular or acicular conductive metal-containing particles which is manifested as a substantial decrease in electrical conductivity of such conductive layers after an overlying transparent magnetic recording layer is applied. For example, surface resistivity values were reported in U.S. Pat. No. 5,382,494 for conductive layers containing 75 weight percent or more granular antimony-doped tin oxide particles dispersed in nitrocellulose, cellulose diacetate, or gelatin as binder measured prior to overcoating with a magnetic recording layer as ranging from 1.times.10.sup.5 to 1.times.10.sup.6 ohms/square for tin oxide dry weight coverages of 3.5 to 12 g/m.sup.2. However, after overcoating with a magnetic recording layer, the surface resistivity values increased to 1.times.10.sup.6 to 1.times.10.sup.9 ohms/square. Similarly, surface resistivity values of 1.times.10.sup.9 to 1.times.10.sup.10 ohms/square were reported in U.S. Pat. No. 5,459,021 for conductive subbing layers containing granular antimony-doped tin oxide particles dispersed in gelatin as binder after overcoating with a magnetic recording layer. Conductive subbing layers containing other granular conductive metal oxide particles, such as zinc antimonate, exhibit similar behavior when overcoated with a magnetic recording layer. For example, conductive layers containing less than 75 weight percent zinc antimonate particles dispersed in a vinylidene chloride-based terpolymer latex as binder are reported in U.S. Pat. No. 5,457,013 to exhibit surface resistivity values of 1.times.10.sup.8 ohms/square for total dry weight coverages of about 0.5-0.65 g/m.sup.2. After overcoating with a magnetic recording layer, the internal resistivity of the conductive layers increased to 1.times.10.sup.10 to 1.times.10.sup.11 ohms/square. Similarly, conductive layers containing 75 weight percent acicular tin oxide particles dispersed in a vinylidene chloride-based terpolymer latex as binder are reported in U.S. Pat. No. 5,719,016 to exhibit surface resistivity values of from 1.times.10.sup.6 to 1.times.10.sup.9 ohms/square for total dry weight coverages of from 0.6 to 0.2 g/m.sup.2. After overcoating with a magnetic recording layer as above, the internal resistivity values of such conductive layers are reported in U.S. Pat. No. 5,731,119 to increase to 1.times.10.sup.8 to 1.times.10.sup.12 ohms/square. Thus, in order to maintain a preferred level of conductivity after overcoating with a magnetic recording layer, conductive layers containing either granular or acicular conductive particles need to contain a higher concentration, a higher weight coverage or both of conductive particles than conductive layers which are not overcoated.
Because the requirements for an electrically-conductive layer to be useful in an imaging element are extremely demanding, the art has long sought to develop improved conductive layers exhibiting a balance of the necessary chemical, physical, optical, and electrical properties. As indicated hereinabove, the prior art for providing electrically-conductive layers useful for imaging elements is extensive and a wide variety of suitable electroconductive materials have been disclosed. However, there is still a critical need for improved conductive layers which can be used in a wide variety of imaging elements, which can be manufactured at a reasonable cost, which are resistant to the effects of humidity change, which are durable and abrasion-resistant, which do not exhibit adverse sensitometric or photographic effects, which exhibit acceptable adhesion to overlying or underlying layers, which exhibit suitable cohesion, and which are substantially insoluble in solutions with which the imaging element comes in contact, such as processing solutions used for photographic elements. Further, to provide both effective magnetic recording properties and effective electrical-conductivity characteristics in an imaging element, without impairing its imaging characteristics, poses a considerably greater technical challenge.
It is toward the objective of providing a useful combination of a transparent magnetic recording layer overlying an electrically-conductive layer containing metal-containing particles dispersed in a polymeric film-forming binder without causing degradation of the physical or electrical properties of the conductive layer that more effectively meets the diverse needs of imaging elements, especially those of silver halide photographic films and thermally processable imaging elements, but also of a wide variety of other types of imaging elements than those of the prior art that the present invention is directed.